"When developing Doom, id Software was aware that many players had tried to create custom levels and other modifications for their previous game, Wolfenstein 3D. However, the procedures involved in creating and loading modifications for that game were cumbersome. John Carmack, lead programmer at id Software, designed the Doom internals from the ground up to allow players to extend the game. For that reason, game data such as levels, graphics, sound effects and music are stored separately from the game engine, in "WAD files". This allowed players to make their own data without any modification to the engine itself."
Description from Wikipedia
In the rush to fill CD-ROMs with collections of shareware, vendors began acquiring Doom WADs at great speed, putting together compilations of levels, along with additional graphics, reviews, documents, and other related files. With the introduction of Doom II and Quake, custom levels, tools and assets continued to be made available and sold on these CD-ROMs until the fervor for CD-ROMs faded in the 2000s. This collection contains many thousands of user-designed levels across many years.